The subject invention relates generally to devices for forming a part from sheet metal stock and, more particularly, to a stretch form die operable with standard drawing die presses that can one hundred percent homogeneously stretch form pre-cut sheet metal blanks or sheet metal advanced from a continuous coil.
Various types of devices exist which permit sheet metal stock to be stretched either prior to or during the forming of the sheet metal stock. In some of these devices, the sheet metal is pre-stretched beyond its elastic limit before the sheet metal is formed by a conventional drawing operation. Such devices permit the metal to be formed while the metal is in a more readily formable state. Examples of such devices are disclosed in U.S. Pat. No. 3,253,448 to Dolney and U.S. Pat. No. 3,314,270 to Dolney.
Other devices combine the stretching and drawing operations so that the resulting part is partially stretch formed and partially formed by drawing. Stretch drawing is commonly used in forming automotive doors, panels, hoods and deck lids wherein the metal is relatively thin in relation to the total area of the part, and the use of conventional drawing operations would result in a highly wrinkled part. U.S. Pat. No. 3,299,689 to Dolney et al discloses one example of such a device.
Still other devices form the sheet metal stock by a combination of stretching and slippage effects. U.S. Pat. No. 2,342,437 to Summers discloses one example of such devices. This device discloses a number of hydraulic clamping cylinders operable to exert graduating different pressures on the workpiece. The clamping cylinders permit varying the amount of clamping and slippage along different sections of the sheet metal as it is being formed by the action of a punch. This device, however, is relatively complicated and is limited in use to those applications where the sheet metal can be formed without the need for co-acting of male and female die halves.
In order to improve upon previous methods for forming sheet metal, applicant developed a process wherein the sheet metal was one hundred percent stretch formed between co-acting male and female die halves. This was accomplished by providing a gripper means which operated to totally lock the entire periphery of the sheet metal during closure of the die halves. The upper surface of the gripper means included a set of five spaced apart beads which extended entirely around the periphery of the female die half while the lower surface of the gripper means was made flat. Each of the beads was provided with relatively sharp edges which bit into the sheet metal when the grippers were closed. This type of gripper means permitted the sheet metal to be homogeneously stretch formed, thus resulting in a higher quality of shape retention in the finished part and also a materials savings. One notable disadvantage, however, which considerably restricted the application of this method was that the footprint left by the grippers was exceedingly large. This meant that a very high stamping pressure needed to be exerted in order to totally prevent any slippage of the metal. As a result, standard sized presses normally used to form parts otherwise suitable for forming by this method could not achieve the necessary clamping pressure.
In accordance with the subject invention, the gripper means has been modified so that it is now possible to homogeneously stretch form sheet metal using conventional standard sized presses now used to form parts, otherwise suitable for this method, by conventional techniques.